Periodic Reporting for period 2 - RELICS (REfining LIgnin by advanced Catalytic schemes powered by Sunlight)
Reporting period: 2023-03-01 to 2024-08-31
Enabling a regenerative economy is key to accelerate the development of a sustainable and less fossil fuel-dependent industry. Lignocellulosic biomass, such as forestry residues and agricultural waste, is widely recognized as a source of commodity chemicals. While current technologies allow us to turn the carbohydrate fraction of such biomass into biofuels, the lignin, the other major component, remains under-exploited owing to the lack of effective valorization strategies to mine the valuable chemicals locked into it.
The importance of lignin relies on its unique composition and abundance; indeed, it is the largest renewable reservoir of aromatics in nature. Bearing in mind that aromatics are crucial building blocks for a wide variety of petrochemicals (plastics, pharmaceuticals, etc.), finding novel routes to source them from lignin holds the key to spark the bioeconomy.
RELICS pursues to establish new approaches to deconstruct lignin into its aromatic units while lying the grounds for a new generation of biorefineries. To do so, in a scalable and fully sustainable fashion, RELICS relies on sunlight to drive the valorization using photocatalysis, i.e. the prevalent strategy when targeting to upscale solar-driven chemistry. Currently, technical barriers and the lack of fundamental insights into these processes are holding back this technology. RELICS deploys an interdisciplinary approach combining materials’ design, interfacial engineering and operando characterization to pioneer a new generation of highly selective photocatalysts with specific end-products. Our ultimate goal of demonstrating a photocatalytic machinery with programmed selectivity and breakthrough yields of lignin conversion will be achieved advancing the project’s core objectives, namely, (1) the rational design of electrocatalysts to afford the selective production of phenolic aldehydes or ketones, (2) the in-depth characterization of the reaction mechanism to rationalize and accelerate the optimization of the (photo)catalysts, and (3) the fabrication of multijunction photocatalysts with enhanced photogenerated carrier utilization and programmed selectivity. Overall, it is foreseen that the scientific outcomes of RELICS will extend beyond the field of biomass valorization and positively impact the fields of organic electrosynthesis and solar-driven chemistry.
The importance of lignin relies on its unique composition and abundance; indeed, it is the largest renewable reservoir of aromatics in nature. Bearing in mind that aromatics are crucial building blocks for a wide variety of petrochemicals (plastics, pharmaceuticals, etc.), finding novel routes to source them from lignin holds the key to spark the bioeconomy.
RELICS pursues to establish new approaches to deconstruct lignin into its aromatic units while lying the grounds for a new generation of biorefineries. To do so, in a scalable and fully sustainable fashion, RELICS relies on sunlight to drive the valorization using photocatalysis, i.e. the prevalent strategy when targeting to upscale solar-driven chemistry. Currently, technical barriers and the lack of fundamental insights into these processes are holding back this technology. RELICS deploys an interdisciplinary approach combining materials’ design, interfacial engineering and operando characterization to pioneer a new generation of highly selective photocatalysts with specific end-products. Our ultimate goal of demonstrating a photocatalytic machinery with programmed selectivity and breakthrough yields of lignin conversion will be achieved advancing the project’s core objectives, namely, (1) the rational design of electrocatalysts to afford the selective production of phenolic aldehydes or ketones, (2) the in-depth characterization of the reaction mechanism to rationalize and accelerate the optimization of the (photo)catalysts, and (3) the fabrication of multijunction photocatalysts with enhanced photogenerated carrier utilization and programmed selectivity. Overall, it is foreseen that the scientific outcomes of RELICS will extend beyond the field of biomass valorization and positively impact the fields of organic electrosynthesis and solar-driven chemistry.
In this first period, the three main research lines proposed in the RELICS project, namely, (1) the fabrication of electrocatalysts and photoelectrochemical systems, (2) the operando characterization of the reaction mechanisms, and (3) the design of photocatalytic systems, have been addressed.
Regarding the electrocatalysis (1), the studies have been focusing on the fragmentation of lignin monomers and dimers, with the aim of accelerating the optimization of the Cα-Cβ fragmentation and the selective oxidation of benzylic alcohols, a critical step in the Cβ-O fragmentation. Our first studies focused on exploring Ni-based catalysts for these reactions, analyzing the effect of its composition on the catalytic response. Preliminary studies in organic solvents appear to be challenging due to the instability of the electrocatalysts, and therefore, the studies were performed in aqueous media. The study revealed that while NiFe-based electrocatalysts were the most active for the oxygen evolution reaction (OER), the incorporation of Co was critical to boost the reactivity towards the oxidation the alcohols and the Cα-Cβ fragmentation. We found that NiCo formulations were the most active towards these reactions showing not only quantitative conversions with >99% selectivity towards the desired products, but also industrially relevant current densities at relatively low applied potentials.
To rationalize the influence of the composition on the catalytic response and the reaction mechanism (2), various spectroelectrochemical tools have been implemented. The results provide unambiguous evidence of the participation of Co in the reaction mechanism, and we have detected the IR signature of various intermediate species by IR.
Regarding the design of photocatalysts (3), the synthesis of CdS and CdSe quantum dots (QDs) using hot-injection and solvothermal methods has been optimized and the protocols to control the particle size and composition have been established. Likewise, various photoredox catalysts have also been tested for the first time in the field. In the case of the CdSe and CdSe, ligand exchange procedures have been optimized to replace the native oleic acid by a wide range of thiol molecules or, even, to strip the organic ligands, but affording stable colloidal dispersions. Photocatalytic test for the conversion of lignin models revealed the strong effect that the ligands impinge on the catalytic conversion with the yields of conversion ranging from <10% to >99%.
Regarding the electrocatalysis (1), the studies have been focusing on the fragmentation of lignin monomers and dimers, with the aim of accelerating the optimization of the Cα-Cβ fragmentation and the selective oxidation of benzylic alcohols, a critical step in the Cβ-O fragmentation. Our first studies focused on exploring Ni-based catalysts for these reactions, analyzing the effect of its composition on the catalytic response. Preliminary studies in organic solvents appear to be challenging due to the instability of the electrocatalysts, and therefore, the studies were performed in aqueous media. The study revealed that while NiFe-based electrocatalysts were the most active for the oxygen evolution reaction (OER), the incorporation of Co was critical to boost the reactivity towards the oxidation the alcohols and the Cα-Cβ fragmentation. We found that NiCo formulations were the most active towards these reactions showing not only quantitative conversions with >99% selectivity towards the desired products, but also industrially relevant current densities at relatively low applied potentials.
To rationalize the influence of the composition on the catalytic response and the reaction mechanism (2), various spectroelectrochemical tools have been implemented. The results provide unambiguous evidence of the participation of Co in the reaction mechanism, and we have detected the IR signature of various intermediate species by IR.
Regarding the design of photocatalysts (3), the synthesis of CdS and CdSe quantum dots (QDs) using hot-injection and solvothermal methods has been optimized and the protocols to control the particle size and composition have been established. Likewise, various photoredox catalysts have also been tested for the first time in the field. In the case of the CdSe and CdSe, ligand exchange procedures have been optimized to replace the native oleic acid by a wide range of thiol molecules or, even, to strip the organic ligands, but affording stable colloidal dispersions. Photocatalytic test for the conversion of lignin models revealed the strong effect that the ligands impinge on the catalytic conversion with the yields of conversion ranging from <10% to >99%.
Recently, photocatalysis has emerged as a promising technology to perform lignin fragmentation. Firstly, the prospects of funneling the solar energy to selectively break the lignin down into its aromatics constituents paves the way to perform the lignin valorization at ambient conditions. Secondly, photocatalysis are consistently identified as the prevalent technology when it comes to upscaling solar driven chemistry. Today’s photocatalysts, however, display low fragmentation yields that render the technology not competitive. RELICS aims to streamline this concept by manufacturing novel photocatalysts with enhanced reactivity and by integrating them into innovative flow photoreactors to achieve maximum yields of fragmentation and continuous operation. RELICS will advance the field of lignin valorization in different aspects. First, by delivering a new generation of photocatalysts based on semiconductor/catalyst assemblies, wherein the semiconductor will be optimized to maximize light harvesting while the catalyst will afford an accurate control of the selectivity of the reactions. Second, by implementing an assortment of spectro(electro)chemical tools to gain unprecedented insights on the lignin fragmentation reaction mechanism, namely, the intermediate species generated, the kinetics of charge transfer, the preferred adsorption sites and molecular configurations, etc. This information will help to unveil the principles that govern the valorization reactions. Third, by providing novel proof-of-concept batch- and flow-type photoreactors for lignin valorization with benchmarking yields of conversion and programmed selectivity.
By the end of the project we expect to achieve the following high-impact goals. Firstly, the demonstration of electrocatalysts that selectively break lignin’s β-O-4 linkages via the Cα-Cβ and Cβ-O bond, respectively. Secondly, the identification of the mechanism whereby the β-O-4 linkage fragmentation occur, by means of operando tools. Thirdly, the demonstration that the selectivity of the photocatalysts can be programmed by coupling electrocatalysts. Fourthly, the demonstration that dual absorbers and concerted photocatalysis affords to achieve higher yields of fragmentation. Fifthly, the validation of flow-type photoreactors for lignin fragmentation.
By the end of the project we expect to achieve the following high-impact goals. Firstly, the demonstration of electrocatalysts that selectively break lignin’s β-O-4 linkages via the Cα-Cβ and Cβ-O bond, respectively. Secondly, the identification of the mechanism whereby the β-O-4 linkage fragmentation occur, by means of operando tools. Thirdly, the demonstration that the selectivity of the photocatalysts can be programmed by coupling electrocatalysts. Fourthly, the demonstration that dual absorbers and concerted photocatalysis affords to achieve higher yields of fragmentation. Fifthly, the validation of flow-type photoreactors for lignin fragmentation.